Positive and negative selection are the critical forces that shape the T cell repertoire as it is formed in the thymus. This grant addresses the molecular mechanisms of positive selection. It makes use of a model system where defined peptide ligands can be used to preferentially induce one selection outcome or the other. Our previous experiments established that positive selection results from a low affinity TCR interaction with self-peptide ligand; while negative selection results from a high affinity interaction. This application presents and tests the hypothesis that weak interactions generate a sustained biochemical signal compared to strong interactions because they fail to invoke negative feedback mechanisms. In order to study the genetic targets of this weak but sustained signal, we will apply gene array analysis of developmentally distinct populations. Because in our model system, positive selection can be induced with specific peptides, the precise kinetics of gene expression changes can be documented. A final goal of this application relates to how an immature T cell becomes desensitized to those weak interactions that rescued it during development. We will study the role of a specific phosphatase in curtailing the signals generated by low affinity ligands, while leaving those generated by high affinity ligands intact. These results will not only deepen our knowledge of the normal differentiation of T cells, but will aid in establishing the rules whereby immune cells recognize and react to "self", a topic that has important implication for autoimmune disease.